Chapter 5 - Chemical Bonding

1. Chapter 5 - Chemical Bonding

2. 1 Chemical compounds A compound is a substance that is made up of two ore more elements combined together chemically E.G. hydrogen gas burning in oxygen will result in formation of a compound of water. 2H2(g) + O2(g) 2H20(l) Compounds can be broken down into their elements. If an electric current is passed through water the compound splits into its elements of hydrogen and oxygen.

3. 2.The octet rule • Elements will try to lose gain or share electrons to achieve 8 electrons in their outer shell. The octet rule states that when bonding occurs atoms tend to reach an electron arrangement with 8 electrons in the outermost energy level • This outermost energy level is also known as the valence shell.

4. Exceptions to the Octet Rule • Transition metals – they can have more or fewer than 8 electrons in outermost energy level • Elements near helium – tend to have 2 electrons in outer energy level rather than 8 in the noble gases.

5. An aluminium atom has the electron structure 2,8,3. It needs to lose 3 electrons to become stable.

6. An oxygen atom has the electron structure 2,6. It needs to gain 2 electrons in its outer shell to become stable.

7. PS. Chemical formulas • NaCl, H2O, CO2 are all compounds • When written as NaCl, H2O, and CO2, they are chemical formulas representing these compounds (like shorthand). • Let’s examine these formulas: • NaCl  This means that there is: • 1 atom of Na ratio • 1 atom of Cl 1:1

8. H2O •  This means that there are : • 2 atoms of H ratio • 1 atom of O 2:1 • CO2 •  This means that there is : • 1 atom of C ratio • 2 atoms of O 1:2 • Fe2O3 (rust) •  This means that there are: • 2 atoms of Fe ratio • 3 atoms of O 2:3

9. 3 Ionic Bonding An Ionic bond is the force of attraction between oppositely charged ions in a compound. Ionic bonds are always formed by the complete transfer of electrons from one atom to another An ion is a charged atom or groups of atom E.G. Na has 11 e- (E.C.= 2,8,1.) When Na gives away this one e- it now has more protons than electrons so it has an overall positive charge. • Ionic bonds generally form between metals and non-metals.

10. Elements in group one lose one e- to from an ion with a positive charge • Group 2 – ion with 2 positive charges etc Note: Positive ions are also called cations Negative ions are called anions

11. How are ionic bonds formed? https://www.youtube.com/watch?v=5IJqPU11ngY

12. Drawing Lewis Structures (Dot and Cross) • Draw each of your elements – fill all shells or just show valence electrons (electrons in outermost shell) • Using Octet rule draw ions of each elements showing the transfer of electrons • Fill in charge of new ions

13. Example

14. Dot and cross diagrams for ionic bonding • E.G. 1 – NaCl. • E.G. 2 – MgCL2

15. Questions Draw dot and cross diagrams for the following • LiF • Na2O • MgO • AlCl3 • MgCl2

16. Crystal lattice structures • Ionic bonds result in a crystal lattice structure • Called the unit cell – repeats itself in all directions (up and down)

17. 4.a Writing Formulas of Ionic Compounds • Usually metal and non-metal • Metals tendency to lose electrons and non metals gain • Ionic compound is neutral overall – need same no of positive and negative charges

18. 4.b Formulas of compounds with group ions

19. Quiz • Nitrate ion • Sulfate ion • Phosphate ion • Ammonium ion • NO3- • SO42- • PO43- • NH4+

20. 4.c Formulas containing transition metals It is not possible to predict the charges of d-block ions They have variable valency

21. Iron Iron – can form FeCl2 or FeCl3, the charge can be represented using roman numerals in the name of the compound Ie FeCl2 iron (ll) chloride

22. Copper Cu2O or CuO • Trend is also seen in manganese and chromium

23. Naming these compounds • -ide compound with 2 elements • -ate contains ocygen and 2 other elements

24. 5. d-Block elements and Transition Elements • Have variable valency • Usually form coloured compounds • Widely used as catalysts A transition metal is one that forms at least one ion with a partially filled d sublevel

25. Uses of ionic materials • Salt tablets are taken to replace lost salt in perspiration. • Brine is used to cure bacon in a preservation process. • Fluoridation of water supplies to prevent tooth decay.

26. 6. Covalent Bonding A covalent bond is a shared pair of electrons. • A single bond has 1 shared pair of electrons.(sigma σ) • A double bond has 2 shared pairs of electrons. (pi π ) • A triple bond has 3 shared pairs of electrons. • E.G. H-H O=O N N • Covalent bonds are typical of non-metal elements. • (Metals mix to form alloys.)

27. · · · · = O O · · · · N = N · · · · Dot and Cross Diagrams for Covalent Bonding O2: N2: The number of electron pairs is the bond order.

28. Examples H2O Cl2

29. NH3 H2

30. CH4

31. Sigma bonds vs Pi bonds Sigma bonds. • A sigma bond is formed when electrons are shared in line with the nuclei. (a head-on overlap of orbitals.) Pi bonds. • A Pi bond is formed when the shared orbitals are side on i.e. (not in line with the nuclei.) N.B. Sigma bonds are stronger. In a covalent single bond it is a sigma bond, however, in a double or triple bond there is 1 sigma bond and the others are pi bonds.

33. Orbital overlap single bond

34. Double bond

35. Bond in oxygen Contains one sigma and one pi bond

36. Draw dot and cross diagrams for… • HCL • CO2 • NO2

37. 7. Characteristics of ionic and covalent bonds We will look at the following headings • Hardness • Melting and Boiling Points • Conduction of electricity

38. Comparison

39. 8. Shapes of Covalent Molecules VSEPR Theory Valence Shell Electron Pair Repulsion Theory The shape of a molecule depends on the number of pairs of electrons around the central atom. • Electrons are negatively charged -> pairs repel each other and want to be as far apart as possible

41. . • The following shapes will arise: Linear – one or two pairs of e- around the central atom. Bond angle is 180º Trigonal planar – Three pairs of e-, bond angle is 120º Tetrahedral– Four pairs of e-, bond angle is 109.5º

42. Loan Pairs and EPR • If un-bonded pairs of electrons remain on the central atom they will distort the shape of the molecule. • Two examples include: • H2O • NH3

43. Answering VSEPR questions • Check valence electron on central atom • How many electrons involved in bonding? • How many lone pairs • Assign shape and bond angle-diagram

44. VSEPR CO2

45. In a bond between two identical atoms the pair of electrons are shared equally • chemists have found that in many bonds the pair of electrons are attracted to one of the atoms more than to the other.

46. H2, Cl2: 9. Electronegativity • Polarity refers to a separation of positive and negative charge. In a nonpolar bond, the bonding electrons are shared equally: • In a polar bond, electrons are shared unequally because of the difference in electron density.

47. Electronegativity Electronegativityis the relative attraction that an atom in a molecule has for the shared pairs of electrons in a covalent bond

49. Hydrogen and Chlorine • Electrons attracted to chlorine more than to hydrogen [ bigger , but more +ve nucleus] • therefore the electrons spend more time near the chlorine than near the hydrogen • this gives the chlorine a slightly negative charge [δ- delta minus] • it gives the hydrogen a slightly positive charge [δ+ delta plus]

50. H Cl